A better understanding of the signal transduction pathways and enzymes underlying disease etiology and pathophysiology has greatly facilitated the search for new therapeutic agents. One important class of enzymes that has been the subject of intensive investigation for targeting disease processes is protein kinases.
Protein kinases are key regulators of cell growth, differentiation, metabolism and function. Protein kinases are a family of structurally related enzymes that are responsible for control of a variety of signal transduction processes within the cell (The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995). Almost all protein kinases contain a catalytic domain consisting of approximately 250 to 300 amino acids. In general, protein kinases mediate their intracellular signaling by catalytic transfer of a γ-phosphoryl group from ATP to target protein substrates. Protein kinases are classified into families by the substrates they phosphorylate. Sequence motifs have been identified that correspond to each of these kinase families such as protein-tyrosine, protein-serine/threonine, and lipids (Hanks, S. K., Hunter, T., FASEB J. 1995, 9 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al., Cell 1992, 70, 419-429). In response to a variety of stimuli, protein kinases allow the cell to make decisions by acting as a molecular “on/off” switch that can either perturb or regulate target protein function.
Abnormal protein kinase-mediated signal transduction in a cell is the underlying cause of many pathophysiological states. These disease states include, but are not limited to, autoimmune disease, allergy and asthma diseases, neurological and neurodegenerative diseases, metabolic diseases, Alzheimer's disease, cardiovascular disease, and cancer. Accordingly, protein kinases are considered rational drug targets for therapeutic intervention and protein kinase inhibitors are thought to be effective therapeutic agents.
The aurora family of serine/threonine protein kinases is essential for cell proliferation (Trends in Cell Biology 9, 454-459 (1999); Nat. Rev. Mol. Cell. Biol. 2, 21-32 (2001); Trends in Cell Biology 11, 49-54 (2001)). The human aurora kinase family consists of three highly homologous kinases (A or “2”, B or “1” and C or “3”). During normal cell proliferation, these proteins are involved in chromosome segregation, mitotic spindle function, and cytokinesis. Aurora kinase expression is low in resting cells and peaks during the G2 and mitosis phases of the cell cycle. Several proposed mammalian substrates for Aurora kinases that are important for cell division include histone H3, TPX2, myosin II regulatory light chain, CENP-A, and protein phosphatase 1.
Since the elucidation of their key role in mitotic progression and cell division, Aurora kinases have been closely linked to tumorigenesis. For example, Aurora kinase gene amplification and overexpression has been reported in many cancers. A coding single nucleotide polymorphism (SNP) has been identified that is significantly more frequent in advanced gastric cancer relative to early stage gastric cancer, and this SNP correlates with elevated kinase activity (Cancer Lett. Jan. 10, 2006). Overexpression of Aurora A induces centrosome amplification, aneuploidy and transformation in rodent fibroblasts (Bischoff, J. R. et al. EMBO. J 17, 3052-3065 (1998); Nat. Genet. October 20 (2):189-93 (1998)). This oncogenic activity is likely due to the generation of chromosome instability. Indeed, there is a strong correlation between Aurora A overexpression and chromosome aneuploidy in breast and gastric cancer. (Int., J. Cancer 92, 370-373 (2001); British Journal of Cancer 84, 824-831 (2001)). Aurora B expression is elevated in cell lines derived from tumors of the colon, breast, lung, melanoma, kidney, ovary, pancreas, CNS, gastric tract and leukemias (Oncol Res. 2005; 15(1):49-57; Tatsuka et al. 1998, 58, 4811-4816; British Journal of Cancer 84, 824-831 (2001); EMBO J. 17, 3052-3065 (1998)). In prostate cancer, increased nuclear expression of Aurora B was observed in high Gleason grade anaplastic prostate cancer tissues relative to low and intermediate grades, and Aurora B expression was accompanied by the phosphorylation of the histone H3 substrate (Prostate 66(3): 326-33 (2003)). Aurora C is overexpressed in primary colorectal cancer (Journal of Biological Chemistry 274, 7334-7340 (1999); Jpn. J. Cancer Res. 91, 1007-1014 (2000)).
Because Aurora kinase inhibition in tumor cells can result in mitotic arrest and apoptosis, these kinases are important targets for cancer therapy. Given the central role of mitosis in the progression of virtually all malignancies, inhibitors of the Aurora kinases therefore are expected to have the potential to block growth of cancers or tumors and have application across a broad range of human cancers or tumors.